module ActiveRecord module Validations module ClassMethods # Validates whether the value of the specified attributes are unique across the system. Useful for making sure that only one user # can be named "davidhh". # # class Person < ActiveRecord::Base # validates_uniqueness_of :user_name, :scope => :account_id # end # # It can also validate whether the value of the specified attributes are unique based on multiple scope parameters. For example, # making sure that a teacher can only be on the schedule once per semester for a particular class. # # class TeacherSchedule < ActiveRecord::Base # validates_uniqueness_of :teacher_id, :scope => [:semester_id, :class_id] # end # # When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified # attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself. # # Configuration options: # * :message - Specifies a custom error message (default is: "has already been taken"). # * :scope - One or more columns by which to limit the scope of the uniqueness constraint. # * :case_sensitive - Looks for an exact match. Ignored by non-text columns (+true+ by default). # * :allow_nil - If set to true, skips this validation if the attribute is +nil+ (default is +false+). # * :allow_blank - If set to true, skips this validation if the attribute is blank (default is +false+). # * :if - Specifies a method, proc or string to call to determine if the validation should # occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The # method, proc or string should return or evaluate to a true or false value. # * :unless - Specifies a method, proc or string to call to determine if the validation should # not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The # method, proc or string should return or evaluate to a true or false value. # # === Concurrency and integrity # # Using this validation method in conjunction with ActiveRecord::Base#save # does not guarantee the absence of duplicate record insertions, because # uniqueness checks on the application level are inherently prone to race # conditions. For example, suppose that two users try to post a Comment at # the same time, and a Comment's title must be unique. At the database-level, # the actions performed by these users could be interleaved in the following manner: # # User 1 | User 2 # ------------------------------------+-------------------------------------- # # User 1 checks whether there's | # # already a comment with the title | # # 'My Post'. This is not the case. | # SELECT * FROM comments | # WHERE title = 'My Post' | # | # | # User 2 does the same thing and also # | # infers that his title is unique. # | SELECT * FROM comments # | WHERE title = 'My Post' # | # # User 1 inserts his comment. | # INSERT INTO comments | # (title, content) VALUES | # ('My Post', 'hi!') | # | # | # User 2 does the same thing. # | INSERT INTO comments # | (title, content) VALUES # | ('My Post', 'hello!') # | # | # ^^^^^^ # | # Boom! We now have a duplicate # | # title! # # This could even happen if you use transactions with the 'serializable' # isolation level. There are several ways to get around this problem: # - By locking the database table before validating, and unlocking it after # saving. However, table locking is very expensive, and thus not # recommended. # - By locking a lock file before validating, and unlocking it after saving. # This does not work if you've scaled your Rails application across # multiple web servers (because they cannot share lock files, or cannot # do that efficiently), and thus not recommended. # - Creating a unique index on the field, by using # ActiveRecord::ConnectionAdapters::SchemaStatements#add_index. In the # rare case that a race condition occurs, the database will guarantee # the field's uniqueness. # # When the database catches such a duplicate insertion, # ActiveRecord::Base#save will raise an ActiveRecord::StatementInvalid # exception. You can either choose to let this error propagate (which # will result in the default Rails exception page being shown), or you # can catch it and restart the transaction (e.g. by telling the user # that the title already exists, and asking him to re-enter the title). # This technique is also known as optimistic concurrency control: # http://en.wikipedia.org/wiki/Optimistic_concurrency_control # # Active Record currently provides no way to distinguish unique # index constraint errors from other types of database errors, so you # will have to parse the (database-specific) exception message to detect # such a case. def validates_uniqueness_of(*attr_names) configuration = { :case_sensitive => true } configuration.update(attr_names.extract_options!) validates_each(attr_names,configuration) do |record, attr_name, value| # The check for an existing value should be run from a class that # isn't abstract. This means working down from the current class # (self), to the first non-abstract class. Since classes don't know # their subclasses, we have to build the hierarchy between self and # the record's class. class_hierarchy = [record.class] while class_hierarchy.first != self class_hierarchy.insert(0, class_hierarchy.first.superclass) end # Now we can work our way down the tree to the first non-abstract # class (which has a database table to query from). finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? } column = finder_class.columns_hash[attr_name.to_s] if value.nil? comparison_operator = "IS ?" elsif column.text? comparison_operator = "#{connection.case_sensitive_equality_operator} ?" value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s else comparison_operator = "= ?" end sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}" if value.nil? || (configuration[:case_sensitive] || !column.text?) condition_sql = "#{sql_attribute} #{comparison_operator}" condition_params = [value] else condition_sql = "LOWER(#{sql_attribute}) #{comparison_operator}" condition_params = [value.mb_chars.downcase] end if scope = configuration[:scope] Array(scope).map do |scope_item| scope_value = record.send(scope_item) condition_sql << " AND " << attribute_condition("#{record.class.quoted_table_name}.#{scope_item}", scope_value) condition_params << scope_value end end unless record.new_record? condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?" condition_params << record.send(:id) end finder_class.with_exclusive_scope do if finder_class.exists?([condition_sql, *condition_params]) record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value) end end end end end end end